Abstract

The study was conducted to design, manufacture and test a household rice
husk gasifier for families in Mekong Delta, Vietnam. The downdraft gasifier
can supply 5,500 kcal/h with a reaction chamber diameter of 0.254 m and
height 0.6 m. The rice husk feed stock rate was 6.1 kg/h; synthesis gas was
generated at 29.2 m3/h. The treatments were: five levels of the
equivalence ratio (ER) of 0.2, 0.25, 0.3, 0.35 and 0.4. ER is defined as the
ratio of the actual air flow rate with the stoichiometric air flow rate,
according to the formula ER = Qac/Qst (the
stoichiometric air flow rate is the air required to burn completely); each
ER was studied at three removal rates of biochar (10, 12 and 14 rpm of the
motor driving the screw that removes the biochar.

The syngas low heating value (LHV) was highest when the equivalence ratio
(ER) was 0.3 at all three rotation levels of the biochar removal system. The
specific gasification rate (SGR) was increased when ER increased. The
highest thermal efficiency was 54% at ER = 0.3 and the rotation of biochar
removal was 14 rpm. The corresponding syngas components were CO 24.0%, C
nHm 0.76%, H2 7.69%, O2 0.82% with
LHV of 1,627 kcal/m3 and SGR 122 kg/h/m2.

Introduction

The conversion technologies for utilizing biomass can be separated into four
basic categories: direct combustion processes, thermochemical processes,
biochemical processes and agro-chemical processes (Alexis 2005). The
thermochemical conversion of biomass (pyrolysis, gasification, combustion)
is one of the promising routes among the renewable energy options.
Gasification as a method of conversion of energy has several advantages over
traditional power generation methods with efficiency from 14 to 25% (Prabir
2010). In the thermochemical conversion technologies, biomass gasification
has attracted the highest interest as it offers higher efficiencies compared
to combustion and pyrolysis. Gasification is a process of conversion of
solid carbonaceous fuel into combustible gas by partial combustion (Luque
and Speight 2015).

Gasification of biomass for energy production has the potential to be a cost
effective and environmentally sustainable technology. Biomass energy is derived
from the plant sources, such as waste from agricultural and forestry processes,
wood from natural forests and industrial, human or animal wastes (Prabir 2010).
So the energy obtained from biomass is a form of renewable energy and it does
not add carbon dioxide to the environment in contrast to the fossil fuels (Zhang
and Zhu 2010; Mofijur et al 2014). Every year more than 15
billion tonnes of CO2 are added to the environment. The increase
in the level of CO2 in the atmosphere is directly associated with
global warming (Koçar and Civaş 2013; Mofijur et al 2015). Many households in Asian continue
to depend on expensive fossil fuel–based sources which are energy
inefficient and pollute the environment (Pode 2015).

Vietnam’s paddy rice production is around 50
million of tones per year and the Mekong Delta produces more than half of the
country’s total production. In terms of rice husk production, 20% of the weight
of rice is rice husk. Thus around 10 million tonnes of rice husks are generated
every year in Vietnam (Hao and Bich 2015).

Materials and methods

Materials

The rice husk from the variety IR50404 was used in this study, sourced from
a rice mill located in Dong Thap province in the Mekong Delta. The moisture
content was 10 %and the bulk density 106 kg/m3.

The average air temperature at the time of the experiment ranged from 30 to
32oC. The air humidity was 68 to 71%.

The composition of the gaseous products from the biomass gasification process
was measured with the Gas board 3100P device from Wuhan Cubic Optoelectronics
Co, Ltd (Figure 1).

Figure 1.
The gasboard 3100P syngas analyzer

Methods

The air flow rates were tested based on the orifice plate. This is a device
used for measuring air flow rate based on the difference in pressure. This
method can be recommended due to its simple manufacture, handy assembly and
low cost. The actual air flow rate is calculated as follows:

Where: Qac: the actual air flow rate, kg/s; α: flow coefficient;
A2: plate hole area, m2; ρkk: density of
air, kg/m3; ΔP = P1 - P2: differential
pressure before and after the disk, Pa; and m = A2/A
1 the ratio between the area of the orifice plate hole and the area of
the tube.

The gasification efficiency is an important factor determining the actual
technical operation, as well as the economic feasibility of using a gasifier
system. A useful definition of the gasification efficiency if the gas is
used for engine applications is: (Cuong et al 2014).

Location and duration

The fabrication of the downdraft gasifier and the experiments were conducted
in the Renewable Energy Laboratory, Nong Lam University, Ho Chi Minh,
Vietnam, from March 2016 to April 2017.

Statistical analysis

Linear equatons were fitted to the data using the Excel program in the
Microsoft Excel software. The fixed "X" variable was the equivalence ratio;
the dependent variables were concentrations of synthesis gas, low heating
value, specific gasification rate and efficiency.

The reaction chamber was made by using inox SUS 304 with diameter 0.254 m
and height 0.6 m, rice husk hopper 6.1 kg, the biochar removal system
connected with the reaction chamber at the bottom. The cyclone gasifier is a
type of entrained-flow bed and acted as a preliminary syngas cleaner with
diameter D = 0.15 m and height H = 0.4 m. The cooling system transferred the
heat from the syngas to the water and from the water to the air, by indirect
transfer.

Figure 3.
The dust filter of syngas

Rice husk biochar was used to remove impurities, to improve gas quality. We
could observed and evaluated the sensation of syngas combustion by swirl
burner.

Influence of ER on syngas composition

To completely burn rice husks, 4.7 kg of air are needed per kg of rice husk.
Burning it with an using 30 to 40% or an equivalence ratio (ER) of 0.3 to
0.4 only of the air needed for combustion will gasify rice husks, which
produces a producer gas. The experiments were conducted to determine the
effect of ER and char removal to syngas composition, specific gasification
rate (SGR) and system efficiency (η).

The ER is defined as the ratio of the actual air flow rate with the
stoichiometric air flow rate, according to the formula.

ER = Qac/Qst

Where: ER: Equivalence Ratio; Qac: the actual air flow rate (m
3/h); Qst: the stoichiometric air flow rate is the air required
to burn completely (m3/h).

According to (Alexis 2005):

0 ≤ ER ≤ 0.2 The process of pyrolysis, the reaction with oxygen begins to
occur.

0.2 ≤ ER ≤ 0.4 Gasification process.

0.4 ≤ ER ≤ 1.0 The process of burning completely.

In this study we selected the ER within the range of 0.2 to 0.4.

The bulk density of both compacted and non-compacted rice husks ranges from
100 to 120 kg/m3; the specific gasification rate was from 100 to
200 kg/h/m2. In this research, the rice husk density of 100 kg/m
3 was selected. The specific gasification rate depends on the rotation
speed of the biochar removal equipment.

This was selected as 10, 12 and 14 rpm. The biochar normally has an energy
content of about 3000 kcal/kg kg and, when burned completely, produces about
15 to 21% ash which is almost 90% silica. The gas produced from the gasifier
has an energy content of about 3.4 to 4.8 MJ/m3. After
gasification, the percentage biochar leaving the reactor was about 32% of
the total volume of rice husks previously loaded.

The content of CO and H2 in the syngas and the low heating value
increased as the equivalence ratio was increased from 0.2 to 0.3 (Table 2),
and then decreased as the equivalence ratio was raised from 0.3 to 0.4.
These changes probably reflect insufficient air to heat the reaction zone at
the low equivalence ratio, and the presence of non-combustible gases such as
CO2 at higher equivalence ratio (Lanh et al 2016; Bich et al 2017).

The specific gasification rate increased linearly (Figure 5) as the
equivalence ratio increased. The gasifier efficiency, defined as the ratio
of energy of the producer gas per kg of biomass to the low heating value of
the biomass material, varied from 38 to 49% and had a maximum value for an
equivalence ratio of 0.3. The low efficiency observed in other ER was
perhaps due to the insufficiency of heat for endothermic reactions, at the
low level, or the excess combustion of rice husk at the high ER.

Figure 5.
Effect of equivalence ratio on SGR and efficiency with
biochar removal rate of 10rpm

The composition of the syngas and the ‘low heating values’ of the producer
gas predicted from experimental observations with biochar removal rate set
at 12 and 14 rpm are shown in Tables 3 and 4 and Figures 6 and 7 and 8 and
9. The trends with increasing ER were similar to when the biochar removal
rate was 10rpm.

Figure 7. Effect of equivalence ratio on SGR and efficiency with
biochar removal rate of 12rpm

Figure 7 shows the effect of equivalence ratio on the specific gasification
rate. It clearly shows that with an increase in the equivalence ratio,
specific gasification rate continuously increases. Higher equivalence ratio
signifies higher air flow rate for a specific rice husk consumption rate.
The specific gasification rate was lower at the rotation of char removal 10
rpm about 124 kg/h/m2 and higher at the rotation of char removal
12 rpm about 129 kg/h/m2.

A higher equivalence ratio represents a higher air flow rate for a specific
rice husk consumption rate which leads to more amount of CO2
production in combustion zone. Figure 8 shown that the conversion of CO
2 to CO depends upon the rate of reactions occurring in the reduction
zone and length of the reduction zone. With an increase equivalence ratio
from 0.2 to 0.3, increased CO2 amount in combustion zone is
converted into CO and H2, and thereby the fraction of CO and H2
increases from 20.2 to 24.0 and from 6.92 to 7.69, respectively. The
increase in CO2 and decrease in CO and H2 fractions
for the equivalence ratio higher than 0.3 represents that CO2
produced in combustion zone is in excess to that of the conversion capacity
of reduction bed.

Figure 9. Effect of equivalence ratio on SGR and efficiency with
biochar removal rate of 14rpm

The effect of equivalence ratio on rice husk consumption rate is shown in
Figure 9. It is found that with an increase in the equivalence ratio, rice
husk consumption rate increases. Specific gasification rate is found to vary
from 106 to 143 kg/h.m2 for an equivalence ratio varying from 0.2
to 0.4, respectively. The increase an equivalence ratio provides more oxygen
to oxidize and higher amount of rice husk would get combusted. The energy
released will increase the rate of drying and pyrolysis. Rice husk
consumption rate increases not only due to a higher combustion rate, but
also due to the enhanced pyrolysis and drying rate.

In summary (Figure 10) it can be seen that: (i) the lower heat value
increases with ER to reach a maximum at a ER value of 0.3, afterwards
declining as ER was raised to 0.4; and (ii) the lower heat value increased
linearly as the biochar removal rate was increased from 10 to 14 rpm.

Figure 10.
Effect of equivalence ratio and biochar removal rate on
the ‘lower heat value’ of syngas produced in a downdraft
gasifier with rice husks as fuel

Conclusions

The downdraft gasifier is suitable for small-scale rice husk gasification
due to its easy fabrication and operation. The quality of producer gas and
the gasification efficiency are output parameters in the downdraft gasifiers
that are affected by some important parameters.

With an increase in equivalence ratio from 0.2 to 0.3 the low heating value
increase but with an increase in equivalence ratio from 0.3 to 0.4 the low
heating value decrease.

With an increase in equivalence ratio or rotation of char removal, the
specific gasification rate continuously increases.